1. Field of the Invention
The present invention relates to a light quantity measuring apparatus capable of properly measuring quantity of light to be measured, which fluctuates periodically.
2. Description of the Background Art
A light quantity measuring apparatus for measuring quantity of light to be measured has generally a sensor for outputting a signal according to the quantity of received light to be measured, and specifies the quantity of measured light from the output of the sensor. However, when such a light quantity measuring apparatus measures light whose quantity periodically fluctuates like light generated by a display, a measurement result fluctuates according to timings of measurement, and a problem occurs such that a stable measurement result cannot be obtained.
To solve such a problem, as described in “VESA Flat Panel Display Standard”, (U.S.A.), p. 172, an output of a sensor is filtered by a low-pass filter having a time constant sufficiently longer than the period of fluctuations in the quantity of light to be measured, thereby preventing the measurement result from being influenced by the periodical fluctuations in the quantity of light to be measured.
By storing charges generated by a sensor for time sufficiently longer than the fluctuation period of the quantity of light to be measured, the measurement result is prevented from being influenced by periodic fluctuations in the quantity of the light to be measured.
Further, as shown in Japanese Patent Application Laid-Open No. 2003-18458, by storing charges generated by a sensor for time which is an integral multiple of the fluctuation cycle of the quantity of light to be measured, the measurement result is prevented from being influenced by periodic fluctuations in the quantity of the light to be measured.
In addition, by repeating measurement of the quantity of light to be measured and obtaining average of measurement results, the measurement result is prevented from being influenced by the periodic fluctuations in the quantity of light to be measured.
In the method of filtering an output of a sensor by a low-pass filter, however, in a case where it is necessary to measure light having different light quantity fluctuation cycles, a plurality of low-pass filters of different time constants have to be prepared and a low-pass filter to be used has to be selected according to the fluctuation cycle of the quantity of light to be measured. Selection of a low-pass filter is complicated and in a case where the fluctuation cycle of the quantity of light to be measured is unstable or in a case where the fluctuation cycle of the quantity of light to be measured is unknown, it is difficult to select a low-pass filter. For such a reason, an output of the sensor is filtered by a low-pass filter having a very long time constant, and time required for measurement is very long.
In the method of storing charges generated by the sensor for time which is sufficiently longer than the fluctuation cycle of the quantity of light to be measured, the time required for measurement is very long. Moreover, in a case of using a high-sensitivity sensor which is becoming common in recent years, saturation of charges occurs during storage of the charges, and proper measurement cannot be performed.
Further, in the method of storing charges generated by the sensor for time which is an integral multiple of the fluctuation cycle of the quantity of light to be measured, in the case where the fluctuation cycle of the quantity of light to be measured is unstable or in the case where the fluctuation cycle of the quantity of light to be measured is unknown, storage of charges cannot be synchronized with the period fluctuations of the quantity of light to be measured, and proper measurement cannot be performed. In the case of using a high-sensitivity sensor which is becoming common in recent years, saturation of charges occurs during storage of the charges, and proper measurement cannot be performed.
In addition, in the method of repeating measurement of the quantity of light to be measured and calculating average of measurement results, measurement is performed a number of times so that a measurement result is not influenced by fluctuations in the quantity of light to be measured between measurements, so that time required for measurement is very long. When an attempt is made to measure the quantity of light whose light emission period is equal to or less than a few percentages of the light emission cycle like light generated by a CRT (Cathode Ray Tube) display or a plasma display, the interval between the measurements become the light emission period, and a large error may occur in the measurement result. There is possibility that such a large error occurs also in a case of measuring the quantity of light generated by a liquid crystal display employing a drive method called black insertion.
The problem of saturation of charges can be solved by reducing light to be measured which reaches the sensor by a neutral density filter. However, when the transmittance of the neutral density filter decreases, it becomes difficult to assure precision of the transmittance and, depending on the quantity of light to be measured, when the neutral density filter is used, repetitiveness cannot be assured. A problem occurs such that, if a neutral density filter is not used, time required for measurement becomes very long.